Terrestrial observation missions carried out by an observation satellite consist in acquiring images of terrestrial areas, that is to say areas located on the surface of the Earth, in response to requests sent by clients. Usually, an observation satellite is placed in a non-geosynchronous Earth orbit so that it can acquire different terrestrial areas over a period of time, depending on its position in the orbit concerned.
The development of the agility of observation satellites has also made it possible to increase the number of terrestrial areas that can be imaged, and these areas may now be, notably, located on either side of the projection of the orbit onto the Earth's surface, by suitable control of the attitude of the observation satellite. Thus, for each position of an observation satellite in its orbit, there is a plurality of possibilities for the terrestrial area to be imaged.
Usually, a work plan, comprising a list of acquisitions to be performed by an observation satellite, is calculated on the ground on the basis of various requests received from clients. The work plan calculated in this way is transmitted by a ground station to the observation satellite. A new work plan is calculated and transmitted to an observation satellite in a regular way, usually on a daily basis.
At the present time, requests for such acquisitions of terrestrial areas are increasing in number, and must be met within decreasing time scales, as a result, notably, of the increasing opening of the imaging market to the general public.
The increased number of observation satellites enables satellite access to a terrestrial area to be provided within a much shorter time. However, this capacity cannot be fully utilized unless the work plan of each observation satellite can also be updated promptly.
In the case of observation missions, the inclination of the orbits concerned is such that the observation satellites pass close to the Earth's poles, and the ground stations are preferably positioned near one of the Earth's poles, usually the North Pole. Consequently, an updated work plan can only be transmitted to a given observation satellite when said observation satellite overflies said ground station, which occurs only once per orbital period at the best, the positioning of the ground station near one of the Earth's poles making it possible to increase the number of orbital periods during which said observation satellite actually overflies said ground station in the course of a day. The orbital periods of existing observation satellites are usually longer than an hour, whereas it is desirable to be able to update the work plan of each observation satellites within a much shorter period of about ten minutes.
Furthermore, the time taken to respond to a request depends not only on the time required to communicate an updated work plan to an observation satellite, but also, notably, on the time required to retrieve the images acquired by the observation satellite in response to this request. Like the updating of the work plan, the retrieval of the acquired images usually takes place near the North Pole, when the observation satellite transmits said acquired images to a ground station. Consequently, in the best case, at least one orbital period elapses between the instant at which the observation satellite receives an updated work plan and the instant at which the images acquired in response to this updated work plan are received by a ground station.